Primary infection with varicella-zoster virus (VZV) causes chickenpox, and reactivation of the virus from latency results in zoster. The purpose of this project is to study the molecular pathogenesis and latency of VZV and to identify targets for novel therapies. We have studied the role of individual VZV genes on latency of the virus in an animal model. The ORF61 gene of VZV encodes a protein that upregulates the expression of other genes and its homolog in herpes simplex virus is important for reactivation from latency. Cotton rats infected with VZV deleted for the ORF61 gene still developed latent infection in the nervous system with the virus and showed expression of latent viral RNA similar to animals infected with wild-type virus. The ORF21 gene of VZV expresses a protein that is located in the virus protein coat and is expressed in neurons (nerve cells) during latent infection in humans. ORF21 RNA has been detected in neurons of both humans and rodents with latent VZV infection. We constructed a virus that was deleted for the ORF21 gene and found that it was unable to grow in cell culture, unless it was grown on a cell line that expressed the ORF21 protein. Surprisingly, a knock-out virus that lacked the ORF21 gene was able to establish a latent infection in cotton rats. Thus, the ORF21 gene is required for growth of the virus in cell culture, but not for establishing latency in animals. In collaboration with investigators at Wyeth Vaccines in New York, we determined the mechanism of action of a non-nucleoside thiourea compound that selectively inhibited the growth of VZV in cell culture. Using electron microscopy we found that growth of VZV in the presence of the compound resulted in incomplete virus particles that lacked viral DNA. We determined that a virus that was resistant to this compound had a mutation in the VZV ORF54 gene that allowed it to grow in the presence of this compound. Thus, the compound inhibits the function of the VZV ORF54 protein, which is important for incorporation of viral DNA into the virus particle.